This invention relates to an electrical protection device, and more particularly, to a dedicated high-voltage, high-energy power source for electrical equipment such as the high-voltage d-c bulk supply in a large computer power system in which the bulk supply serves as a source for a number of d-c to d-c converters or switching regulators.
In a typical power system for a large computer the bulk supply may comprise a 300 VDC bus obtained by rectification of the three-phase 208 volt a-c utility supply voltage. To provide protection for the computer hardware against malfunctions due to voltage dips commonly occurring in the utility supply voltage, large electrolytic capacitors are usually connected across the terminals of the d-c bus. These capacitors may provide thousands of microfarads of storage as needed to carry the equipment operation through voltage dips with durations of 100 milliseconds or more. At 300 volts d-c very high and potentially dangerous energy levels are involved.
A primary consideration in the design of such a bulk supply is the safety of the supply. The utmost care must be taken to prevent harm to service and operating personnel and to prevent fire hazards in the event of insulation break-down or short circuits produced by hardware defects or human error.
The safe design of such equipment is complicated by several factors. First, is the nature of the energy hazard itself which has a capability of delivering hundreds of amperes of fault current through a short circuit. Fault currents of such magnitudes can produce arcing, showers of molten metal and fire. A second factor is the lack of adequate isolation of the bulk supply from equipment ground. In most equipment the d-c bulk voltage is obtained by direct rectification of the a-c line without the benefit of an isolation transformer. Because the a-c line is referenced to ground by direct connection, high energy fault currents are delivered to the short circuit by the low impedance line. In other equipment an isolation transformer is used but one of the d-c terminals is then connected directly to ground. If the other terminal is shorted to ground in such an arrangement the energy stored in the capacitor is discharged into the short circuit together with fault current delivered by the transformer. Because the 300 volt supply is typically distributed within the equipment cabinet to provide power to switching regulators at remote locations, the 300 volt distribution cables represent a significant hazard and they must be carefully protected against abrasion and various other forms of damage that might produce a short to ground.
In the present invention the d-c bulk supply is isolated by means of a transformer and is then referenced to ground through a high impedance in a manner that significantly reduces the energy hazard for the equipment during its initial test as well as during normal operation or field service activities.